1. Field of the Invention
The present invention relates to a device for stapling sheets, and to an image forming apparatus equipped with this device, that effects stapling of the sheets by suitable bending of a staple in accordance with the stapling thickness of the sheets.
2. Description of the Related Art
A staple, e.g., so-called Hotchkiss, is widely used as a simple method of binding sheets, such as for example printed sheets or copies. There are various devices for carrying out this process, from simple hand-operated tools to devices using a motor. Their action consists in moving a staple pressing member and an anvil towards each other, so that a staple which is positioned between them is pressed and formed by bending so as to pierce and fix the sheets.
FIG. 15 shows a commonly used anvil 101. This is provided with recesses 102 and 103 arranged in a straight line and having a separation and a shape of their curved bottom surfaces matched to the staple. By this means, the staple is bent inwards so that stapling is achieved as the staple tips are brought up close to each other.
Thus staples must be used in stapling that match the thinkness of the sheaf of sheets that are to be stapled. As shown in FIGS. 16A and 16B, when staples 105, 106 are used that match the thicknesses of the sheaf of sheets to be stapled t1 and t2, suitable pressing margins a and b are formed. However, if, as shown in FIG. 17A, staple 105 is too large in relation to stapling thickness of the sheaf of sheets t1, the portion c that is bent over to provide the pressing margin is too long, with the result that the tips of the staple come into contact with each other. The staple therefore bows outwards. If, on the other hand, as shown in FIG. 17B, staple 105 is too short in relation to stapling thickness of the sheaf of sheets t2, the portion d that is bent over to provide the pressing margin is too short, with the consequence that it cannot be properly pressed over. This leads to a failure to grip the sheets properly, with the result that the sheets may become detached during handling.
In recent years, a stapling device attached to an electronic copier have been provided wherein after copying, the copy sheets are stacked in a sorter so that stapling can be performed on a prescribed number of sheets at a time.
In fact, there is a wide range in the total thickness of stacks of sheets to be stapled, from a few sheets to a fairly thick stack. All these are processed by the same stapling device, so sheets do become detached when the stack is too thick for the staples used. This can be prevented by using staples which are on the large side, but if they are too large, as described above, their tips come into contact resulting in bowing making the stapled sheaf of sheets difficult to handle.